FIG. 6 is a circuit block diagram showing, as an example, an arrangement of a conventional high frequency receiving device. A high frequency signal received at an antenna 1 is passed through an LNB (Low Noise Block down converter) 2 and a variable gain amplifier circuit 3, frequency-converted into base-band signals by mixer circuits 4, 5, and passed through variable gain amplifier circuits 8, 9. Then, undesirable high frequency components are removed from the signals using low pass filters (“LPFs”) 10, 11 respectively. The results are again amplified by the amplifier circuits 13, 14 to produce outputs.
The local oscillating signal fed to the mixer circuits 4, 5 is supplied by a local oscillation circuit 15 (the local oscillating signal fed to the mixer circuit 5 is passed through a phase shifting circuit 6). Here, a phase-locked loop (“PLL”) 16 will be described as a circuit which is made up of a typical PLL and communication means which exchanges signals with a controller controlling the PLL. The PLL 16 divides the frequency of the signal output from the local oscillation circuit 15 (oscillating signal from a voltage-controlled oscillation circuit (“VCO oscillating signal)) and compares the result with a signal output from a reference oscillation circuit 19, so as to produce and supply an oscillation frequency control signal to the local oscillation circuit 15. This enables control of the oscillation frequencies of a voltage-controlled oscillation circuit (“VCO”) 21 and a VCO 22 in the local oscillation circuit 15. Communication signals between the PLL 16 and the controller 17 carry both oscillation frequency setting information and VCO switching information.
The local oscillation circuit 15, when designed to include multiple VCOs, is preferably arranged to be variable over a wide range of bandwidth with a minimum number of VCO, in view of irregularities in frequencies caused by VCO parameters.
To this end, two VCOs (VCOs 21, 22) are used in the conventional high frequency receiving device, with a switching point (switching frequency) between the VCOs specified approximately to the middle of the reception bandwidth of the receiving device. Further, by specifying the VCO oscillation frequency ranges in such a fashion as to cover the VCO switching point frequency even if the VCO oscillation frequencies shift due to irregularities of components, it is unambiguously determined depending on receiving frequency which of the VCOs becomes active. No particular problem therefore arises as to how to control VCO switching.
Meanwhile, to correct a shift in cutoff frequency due to component irregularities of the LPFs 10, 11, a reference filter circuit 20 is provided which has equivalent properties to the LPFs 10, 11 through which an actual signal is passed. A shift in cutoff frequency is detected making use of the phase properties of the reference filter circuit 20; the cutoff frequency control circuit 12 controls/adjusts the cutoff frequencies of the LPFs 10, 11 based on the result of the detection. This method does not require any particular control of the adjustment action, since the cutoff frequencies are constantly adjusted during reception.
The conventional technique has two major problems. One of them is that the conventional high frequency receiving device needs an oscillation frequency control voltage as high as about 30 V to ensure a wide frequency range with the VCO. Recent electronics increasingly operate on relatively low voltages and would have to accommodate a dedicated power source to provide the 30 V voltage, which could be a factor for undesirable extra cost.
The other problem is that since the LPFs constantly adjust the cutoff frequencies by means of a reference filter circuit, the conventional high frequency receiving device is bulky and current consuming, as well as suffers from a signal from the reference filter circuit which undesirably finds a path and acts as noise on a base-band signal.